Photodegradation-absence

Photolysis of PVC in the presence of oxygen causes oxidation of the polymer. However, under most (perhaps all) conditions, in both the presence and absence of oxygen, the photodegradation is complicated by scissions of carbon-chlorine bonds. Such scissions may lead to the formation of conjugated polyene sequences via sequential dehydrochlorination (Equation 1). The polyenes... 

Experiments done in the absence of an external stress showed that the effects of degradation crosslinking are significant at relatively short times of UV exposure, and confirmed that the photodegradation is essentially in the surface layers. The oxidized layer thickness appeared to remain more or less constant after a certain exposure. 

Substrate Spectra. Photodecomposition of five insecticidal chemicals stimulated by protease-liberated flavoprotein was studied and results are shown in Tables II and III and Figures 3, 4 and 5. Generally the flavoprotein(s) was significantly more active in stimulating the photodegradation process in the absence than in the presence the flavin cofaggor (FMN). 

With respect to C-parathion and Cl-toxaphene, protease-liberated flavoprotein was significantly more active than phosphate buffer in photodegrading these chemicals to ater-soluble products (Tables II and III). The amount of C-water-soluble products formed from parathion was 5-7 times greater in the presence than in the absence of flavoprotein. It should be noted that the presence of FMN in the mixture caused a slight grange in amount of water-soluble products formed (Table II). 

It is clear from Figure 3 that TX-20 was by far the most active in stimulating DDT photolysis to yield TDE as a major product. Addition of FMN to TX-20 results in inhibition of the photodegradation processes judging by the amount of original DDT recovered in ether extract and detected by GLC (B, Fig. 3). DDT in the phosphate buffer alone with or without FMN, (i.e. in the absence of added flavoproteins) seems to be rather stable. However, small amounts of peaks 1, 2 and 3 were detected under these conditions (C and D, Fig. 3). 

Degradation of DDT, dieldrin and lindane by the flavoprotein preparation was almost more efficient in the absence than in the presence of FMN (e.g. Figure 3). On the contrary, photodegradation of mexacarbate was greatly enhanced by FMN and other flavin cofactors. It is well known that flavin cofactors, such as FMN, are active photosensitizers. Hence it is possible that the mechanisms or pathways involved for the photodegradation of DDT, dieldrin and lindane and that for mexacarbate are different... 

Photolytic. The photodegradation rate of amitrole in water increased in the presence of humic acid. Under simulated sunlight, the half-life of amitrole in water containing 100 mg/L of humic acid-potassium salt was 7.5 h. Degradation did not occur in the absence of humic acid (Jensen-Korte et al, 1987). Direct photolysis of amitrole is not expected to occur because the herbicide shows little or no absorption at wavelength greater than 295 nm (Gore et al., 1971). 

In the absence of cosolutes, the photodegradation rates depended on the orientation of the nitro group. Thus 1-nitropyrene decayed relatively fast by the nitro-nitrite primary intramolecular photorearrangement process, followed by secondary radical reactions. However, 2-nitropyrene and 2-nitrofluoranthene were stable toward photolysis, consistent with the N02 group being in the same plane as the aromatic rings. 

The photodegradation of an aqueous solution of terbuthylazine was not only accelerated, but was also more extensive in the presence of humic acids isolated from soil (Mansour et al., 1997). In the absence of humic acids, only hydroxyterbuthylazine (OBET) was formed (Sanlaville et al., 1996), whereas in the presence of humic acids, dealkylated products (CBAT, CBDT, CEAT, CAAT, OAAT) were formed (Table 23.2) (Sanlaville et al., 1996 Mansour et al., 1997). In contrast, fulvic acids isolated from stream water slowed the photolysis of terbuthylazine, most likely reflecting differences in structure between the soil- and stream-derived materials. The photodegradation of atrazine and its initial photoproduct OEIT (Table 23.2) in artificial sea water containing humic acids was also accelerated compared to photolysis in distilled water (Durand et al., 1990,1991). 

One should bear in mind, however, that photodegradation of cis-1,4-polybutadiene in air carried out in the absence of a photosensitizer proceeds as a free radical process [II]. 

Shirayama H, Tohezo Y, Taguchi S (2001) Photodegradation of Chlorinated Hydrocarbons in the Presence and Absence of Dissolved Oxygen in Water, Wat. Res. 35, No. 

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